By Robert Piggott
BBC religious affairs correspondent
The Roman Catholic Church has attacked the Spanish government's plans to introduce gay marriage, comparing them to releasing a virus into society.
Spain could join other European countries in allowing same-sex unions
The government says it expects homosexual people to be able to marry as early as next year.
The legislation highlights the steep decline in the power and authority of the Church in Western Europe.
This is especially the case in Spain - which was until recently one of Europe's most devout countries.
The bill allowing same sex marriage - expected to be passed by the Spanish cabinet this week - has prompted a harsh response from Roman Catholic bishops.
Their spokesman likened gay marriages to a counterfeit currency.
"It would impose on society a virus, something false, which will
have negative consequences for social life," Juan Antonio Martinez
Socialist Prime Minister Jose Luis Rodriguez Zapatero took office in April, intending to remove what he called the Church's undeniable advantages and create a secular state with streamlined divorce and relaxations in abortion law.
The changes have distressed and outraged the Church, whose influence on Spaniards has declined precipitously since the death in 1975 of the dictator General Francisco Franco. His regime was closely linked to the Church.
Opinion polls suggest that nearly half of Spaniards now almost never go to mass.